1. Technical Field
The present disclosure relates to wireless networks, and, more particularly, to managing transmission power between mobile stations and base stations.
2. Description of Related Art
a. Cellular Wireless Networks Generally
Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are often referred to as “1×RTT networks” (or “1× networks” for short), which stands for “Single Carrier Radio Transmission Technology.” These networks typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Mobile stations typically conduct these wireless communications with one or more base transceiver stations (BTSs), each of which send communications to and receive communications from mobile stations over the air interface. Each BTS is in turn communicatively connected with an entity known as a base station controller (BSC), which (i) controls one or more BTSs and (ii) acts as a conduit between the BTS(s) and one or more switches or gateways, such as a mobile switching center (MSC) and/or packet data serving node (PDSN), which may in turn interface with one or more signaling and/or transport networks.
As such, mobile stations can typically communicate with one or more endpoints over the one or more signaling and/or transport networks from inside one or more coverage areas (such as cells and/or sectors) of one or more BTSs, via the BTS(s), a BSC, and an MSC and/or PDSN. In typical arrangements, MSCs interface with the public switched telephone network (PSTN), while PDSNs interface with one or more core packet-data networks and/or the Internet.
Service providers have also introduced mobile stations and wireless networks that communicate using a CDMA protocol known as EV-DO, which stands for “Evolution Data Optimized.” EV-DO networks, operating in conformance with one or more releases and/or revisions of industry specification IS-856, such as Release 0 and Revision A, both of which are hereby incorporated herein by reference, provide high rate packet-data service (including Voice over IP (VoIP) service) to mobile stations using a combination of time-division multiplexing (TDM) on the forward link (from the network to mobile stations) and more conventional CDMA technology on the reverse link (from mobile stations to the network). Furthermore, some “hybrid” mobile stations can communicate with both 1× networks and EV-DO networks.
In the EV-DO context, a mobile station is typically referred to as an access terminal, while the network entity (i.e., the base station) with which the access terminal communicates over the air is known as an access node, which typically includes a device known as a radio network controller (RNC), which is similar to a BSC in 1× networks. The access node also includes one or more BTSs, each including one or more antennas that radiate to define wireless coverage areas such as cells and sectors. Note that sectors are used in the balance of this written description as an example of a wireless coverage area, though this is for explanation and not to the exclusion of cells or other coverage areas. Among other functions, the RNC controls one or more BTSs, and serves as a conduit between the BTSs and a PDSN, which provides access to a packet-data network. Thus, when positioned in a sector provided by an access node, an access terminal may communicate over the packet-data network via the access node and the PDSN.
b. Reverse-Link Power Control
i. The Power Control Bit (PCB) and the Ratio Eb/Nt 
In CDMA networks, the transmitting power of a mobile station on the reverse link of a traffic channel at any given moment is based on a number of mechanisms, two of which are known as the power control bit (PCB) and the ratio Eb/Nt. The PCB is a bit (0 or 1) that the base station sends to the mobile station on the forward link quite frequently, on the order of 800 times per second (i.e., once every 1.25 ms). The mobile station repeatedly responsively adjusts its transmission power to the base station on the reverse link. Typically, if the base station sends a PCB equal to 0, the mobile station will decrease the power by a set decrement, such as 1 dB, whereas if the base station sends a PCB equal to 1, the mobile station will increase the power by a set increment, which may also be 1 dB. Using these example numbers, the mobile station's reverse-link transmission power would change by plus or minus 1 dB every 1.25 ms.
Each such 1.25-ms cycle, a typical base station determines whether to transmit a PCB equal to 0 or 1 to a given mobile station by comparing (i) a signal-to-noise ratio that the base station computes for that mobile station with (ii) a stored threshold value for that signal-to-noise ratio that the base station maintains on a per-mobile-station basis. This ratio is known as and referred to herein as Eb/Nt, while the threshold is referred to herein as the “Eb/Nt setpoint.”
Eb/Nt is a ratio of (i) the strength (Eb for “energy per bit”) at which the base station receives the reverse link from the mobile station to (ii) the strength (Nt for “noise”) at which the base station is receiving signals from all other sources on the sector/carrier on which the mobile station is operating (where a sector/carrier is an instance of a given carrier frequency on which the base station provides service in a given sector, though coverage areas other than sectors could be used in various implementations). Eb/Nt, then, is a signal-to-noise ratio for the reverse-link part of a traffic channel. As stated, the base station typically computes Eb/Nt at the same frequency at which it transmits the PCBs, which again may be once every 1.25 ms.
In typical operation, then, for a given mobile station (and in fact for each mobile station the base station is serving), every 1.25 ms, the base station compares the most recently computed Eb/Nt for that mobile station with the Eb/Nt setpoint for that mobile station. If the Eb/Nt exceeds the setpoint, the base station is receiving a strong enough signal from the mobile station, and thus the base station transmits a PCB of 0, causing the mobile station to decrement its reverse-link power. If, however, the Eb/Nt is less than the setpoint, the base station is not receiving a strong enough signal from that mobile station, and thus the base station transmits a PCB of 1, causing the mobile station to increment its reverse-link power.
Thus, a mobile station's reverse-link transmission power typically stabilizes on the traffic channel after some time to a point that achieves an Eb/Nt (as measured at the base station) that is near the mobile station's Eb/Nt setpoint, which can be changed during operation. Furthermore, a network typically operates using configurable parameters corresponding to initial, minimum-allowed, and maximum-allowed values for the Eb/Nt setpoint for each mobile station, further shaping the reverse-link transmission-power behavior of the mobile stations.
ii. Reverse-Link Frame Error Rate (RFER)
Using 1× networks by example, data is transmitted on the air interface in units known as frames, which typically last 20 ms. Some reverse-link frames received by the base station contain no errors, while some frames do contain errors as a result of imperfect transfer from the mobile station, and some are not received at all. The reverse-link frame error rate (RFER), then, is a ratio, computed per mobile station by the base station over a given time period of (i) the number of error-containing and missing frames from each mobile station to (ii) the total number of frames that the base station should receive from that respective mobile station. Other things being more or less equal, the more power the mobile station uses to transmit to the base station, the lower the mobile station's RFER will be.
At approximately the same frequency at which the base station is receiving reverse-link frames (i.e., once every 20 ms) from a mobile station, the base station computes a RFER for that mobile station over some previous number of frames, e.g., 20, 100, 200, etc. Thus, on a frame-by-frame basis, the base station computes a RFER for some rolling window of previous frames. Furthermore, each time the base station computes the RFER for that respective mobile station, the base station compares that computed value with a threshold: a parameter often and herein referred to as the “RFER target,” which may be around 2%.
If the RFER for that mobile station exceeds the RFER target, the base station is receiving too many error-containing frames and/or missing too many frames from that mobile station, and thus the base station will responsively increase the Eb/Nt setpoint for that mobile station. In the short term, this will result in the base station's computed Eb/Nt for that mobile station falling below the new, higher setpoint, which in turn will result in the base station repeatedly sending PCBs of 1 to the mobile station. This, in turn, will result in the mobile station increasing its reverse-link transmission power, which will then typically stabilize at a level that will result in the base station computing an Eb/Nt for that mobile station that is close to the new, higher Eb/Nt setpoint, and perhaps result in an acceptable RFER for that mobile station.
If, on the other hand, the RFER falls below the RFER target, the mobile station may be using excessive power on the reverse-link—in essence, the base station may be receiving a signal from that mobile station that may be considered too strong, perhaps at the expense of that mobile station's battery life, and perhaps creating excessive noise on the sector/carrier. If that occurs, and perhaps only if it holds for a specified period of time, the base station may decrease the Eb/Nt setpoint for that mobile station, resulting in the short term in the Eb/Nt computed by the base station repeatedly exceeding the new, decreased setpoint, resulting in the base station repeatedly sending PCBs of 0 to the mobile station. This will result in the mobile station decreasing its reverse-link transmission power, which should then stabilize at a level that will result in the base station computing an Eb/Nt that is close to the new, decreased Eb/Nt setpoint.
The base station's repeated calculation of the RFER for a mobile station and comparison with the mobile station's current RFER target results in the base station iteratively adjusting the Eb/Nt setpoint for that mobile station. In turn, the base station's even-more-frequent calculation of the mobile station's Eb/Nt and comparison with the mobile station's current Eb/Nt setpoint causes the base station to iteratively send PCBs of 0 (for less power) or 1 (for more power) to the mobile station, which cause the mobile station to accordingly adjust its reverse-link transmission power on the traffic channel. This calibration process is conducted in an attempt to keep the RFER calculated by the base station and associated with the mobile station at or just below what is deemed to be an acceptable threshold, which again may be around 2%.
Different situations may occur on a sector/carrier at different times. For one, the number of mobile stations using traffic channels can vary between just a few, such as 10, to a larger number, such as 30, and perhaps approach an upper bound (e.g., 61 assuming 1×RTT CDMA RC3). And, as stated, the power that the mobile stations use for transmission to the base station can vary. That is, variables such as terrain, weather, buildings, other mobile stations, other interference, and distance from the base station, among others, can affect the RFER that the base station measures for a given mobile station, and thus the amount of power the mobile station uses on the reverse link. Using too much power can drain battery life, and it may sometimes be the case that a mobile station reaches its maximum transmission power and still cannot achieve an acceptable RFER, in which case it may not be able to communicate with the base station.
Note that, in some implementations, a ratio other than Eb/Nt may be used. In particular, each mobile station, when operating on a traffic channel, may also transmit on the reverse-link on what is known as a reverse pilot channel. The base station may then compute a ratio known as Ec/Io for that mobile station, which would be a ratio of (i) the power level (“Ec” for “energy per chip”) at which the base station is receiving the reverse pilot channel and (ii) the power level (“Io”) at which the base station is receiving all transmissions (including the reverse pilot channel) on the sector/carrier on which the mobile station is operating. The base station would then operate with respect to Ec/Io as described above with respect to Eb/Nt. And certainly other power-control mechanisms are used in the art.
iii. Reverse Noise Rise (RNR)
As stated, in general, interference can be—and often is—present on the reverse link of a sector/carrier, as a base station will receive transmissions not only from mobile stations that are operating on that sector/carrier, but will also often receive transmissions on that frequency from other mobile stations, other devices, and/or any other sources of interference on that frequency in that area. At any given moment, the sum total of what a base station is receiving on a sector/carrier—from mobile stations operating on that sector/carrier, as well as from all other sources—is known as the “reverse noise” on the sector/carrier.
Quite frequently (e.g., once per frame (each of which may last 20 ms)), base stations compute a value known as “reverse noise rise” (RNR) for a given sector/carrier, which is the difference between (i) the reverse noise that the base station is currently detecting on the sector/carrier and (ii) a baseline level of reverse noise for the sector/carrier. Thus, the base station computes how far the reverse noise has risen above that baseline.
For the baseline level, CDMA networks may use a value such as the lowest measurement of reverse noise on the sector/carrier in the previous 24 hours, or perhaps an average of the 24-hour lows over the previous week, or some other suitable value. And some networks, including EV-DO networks, may periodically use what is known as a silent interval, which is a coordinated time period during which mobile stations do not transmit anything to the base station. The base station can then measure whatever else is out there. In that case, the baseline level would correspond to the amount of reverse noise that is present when the sector/carrier is unloaded. And other reverse-link-noise levels could be used as a baseline.
Other things being more or less equal, the lower the RNR is at a given moment, the more favorable the RF environment is for communication between mobile stations and the base station at that time. As one would expect, the higher the RNR is at a given moment, the less favorable the RF environment is at that time. Also, a low RNR generally corresponds to a sector/carrier being lightly loaded, in other words that is supporting communications for a relatively low number of mobile stations. A high RNR, again as one might expect, generally corresponds to a sector/carrier being heavily loaded, in other words that is supporting communications for a relatively high number of mobile stations.